Chemical nickel plating



Jan. 13, 1970 PLATING RATE MILS/HOUR PLATING RATE MlLS/HOUR w. R.VINCENT ET AL CHEMICAL NICKEL PLATING Filed Aug. 4, 1966 (HOURS) PLATINGTIME v I l 2 3 PLATING TIME (HOURS) INVENTORS Kali/1k??? if M1 268)?! BY525 010 W5 Qarasd H T TORNE) United States Patent 3,489,576 CHEMICALNICKEL PLATING William R. Vincent, Birmingham, and Slavko M. Dobrash,

Warren, Mich., assiguors to General Motors Corporation, Detroit, Mich.,a corporation of Delaware Filed Aug. 4, 1966, Ser. No. 570,241 Int. Cl.C23c 3/02, 5/00; C09d 5/00 U.S. Cl. 1061 9 Claims ABSTRACT OF THEDISCLOSURE In electroless nickel plating baths, plating rate stabilizersselected from the group consisting of 1,3-diisopropylthiourea andthiobenzanilide.

This invention relates to nickel plating and, more particularly, to thechemical deposition of nickel from an aqueous solution containing asoluble nickel salt and a suitable reducing agent, such as ahypophosphite compound.

Basically, such deposition involves the reduction of nickel ion tonickel metal by the hypophosphite ion, or probably by the reactionproduct of the hypophosphite ion with water. The deposition is catalyzedat least initially by the metal of the workpiece and thereafterautocatalyzed by the nickel metal which is plated. The depositionreaction is represented by the equation:

Electroless nickel plating has found a number of useful applications andit has certain advantages over other methods of plating, such aselectrodeposition, chiefly because it provides an extremely uniformplating thickness regardless of the shape of the workpiece; thusuniform, dense, nonporous plate of high quality can be attained onworkpieces of highly complex contour. Some of the disadvantages of theelectroless nickel plating process have been the relatively slow rate ofdeposition and the tendency for the bath to decompose. The first ofthese problems has been solved, to some extent, by the addition of lessthan 0.0005 g./l. of certain thiocompounds, such as thiourea, orpotassium thiocyanate, which increase the plating rates of the baths.The second of these disadvantages has also been solved, to some extent,by the addition of less than 0.2 g./l. of thiocompounds such as thioureaamong others. Hence, certain thiocompounds in certain concentrationshave been used in electroless nickel baths by the prior art for one orthe other reason. However, the particular thiocompounds used heretoforeand the concentrations employed have not solved another major problemthat still exists with these baths. That problem is the stability of theplating rate. By plating rate stability, we mean that property of thebath which permits it to deposit nickel at substantially the same rateover a given period of time, preferably at least 4 hours or theequivalent of half a work shift. Baths which plate at substantially thesame rate over a reasonable period of time before needing to bediscarded obviously lend themselves more readily to productionapplications than do those which have variable or decreasing platingrates. Such variable/decreasing plating rate baths, which are typical ofthe prior art, need frequent changing amounting to a considerable amountof production downtime.

3,489,576 Patented Jan. 13, 1970 It is, therefore, an object of ourinvention to both increase and stabilize the plating rates ofelectroless nickel baths.

FIGURE 1 is a graphical representation of the performance of bathswithin the scope of our invention.

FIGURE 2 is a graphical representation of the performance of baths otherthan those of our invention.

Briefly stated, our invention comprises the addition of a compound fromthe group consisting of thiobenzanilide and 1,3-diisopropylthiourea toan electroless nickel bath to both increase and stabilize the platingrate of the bath over a fixed period of time. More specifically, ourinvention comprises the addition of small, but effective, amounts butless than 0.047 g./1. of the former and less than 0.009 g./l. of thelatter.

Though the additives of our invention may be used in conjunction withany hypophosphite type electroless nickel deposition bath, I prefer touse them in conjunction with a bath and process of the type disclosedand claimed in U.S. Patent 2,876,116 Jendrzynski, filed Dec. 29, 1955,and assigned to the assignee of this invention. The aforesaid patent isdirected toward the addition of less than 0.02 g./l. of molybdenum tostabilize electroless nickel deposition baths. In this connection, then,we prefer to use the additives of our invention in conjunction with abath comprising about 11.8-22.3 g./l. of nickel sulfate hexahydrate,11.8-22.3 g./l. of sodium hypophosphite monohydrate, 6.7 ll7.2 g./l. ofacetic acid, 0.0088- 0.0ll6 g./l. of molybdenum, and sodium hydroxide toa pH of 5.5. We prefer to add the molybdenum to the solution by addingthereto 0.0871.14 mL/l. of an molybdic acid solution. Within the rangeof the bath recited above, optimum results, in terms of plating rate,have been obtained using a solution comprising the 11.8 g./l. of nickelsulfate hexahydrate, 22.3 g./l. of sodium hypophosphite monohydrate,9.34 ml./l. of acetic acid, 0.88 ml./l. of an 85% molybdic acidsolution, and sodium hydroxide to a pH of 5.5. Obviously, as is known inthe art, other compounds may be employed in making up a bath to be usedin our invention. In this connection, any of the commonly known solublenickel salts may be employed. These include nickel chloride, nickelsulfate, and nickel acetate. The concentration of the nickel ion shouldbe from l-15 g./l. The hypophosphite ion may be added in the form ofhypophorous acid, sodium hypophosphite or other soluble hypophosphites.The hypophosphite ion concentration should be from about 2-40 g./l. pHin the range of 4.5-6.0 may be employed with adjustment thereof by theaddition of sodium hydroxide. As is the normal practice, a suitablebuffer may be used in order to adjust and maintain the proper pH. Theamount of buffer used will, of course, depend upon its molecular weightand the number of effective carboxyl groups per molecule, since from 1.5to 2.5 effective carboxyl groups per nickel ion will provide effectivebuffering. When acetic acid or propionic acid is used, from 230 g./l.are generally sufficient.

In some instances it may be desirable to also include in the bath achelating agent such as glycine, glycolic acid,ethylenediaminetetraacetic acid, and the like, particularly where thenickel ion concentration is high. The

chelating agent functions to maintain thenickel in solution by forming asoluble complex ion with at least a portion of the nickel. Somecompounds, for example glycine, serve not only as a chelating agent, butalso as a buffer to maintain the desired pH.

To the above-identified baths, among others, is added and the platingrate deviation, but the aforesaid narrower concentration ranges producedoptimum results. By plating rate deviation, we mean the change inplating rate (i.e., dY/dX) that occurs over the 4-hour period whencompared to the initial plating rate. By improving the deviation, weminimize the magnitude of this change. 1,3-diisopropylthiourea in theconcentrations indicated increased the total deposit 95% over thatobtained from the standard solution not containing the additive over thesame 4-hour plating time. Thiobenzanilide in the concentrationsindicated increased the total deposit 84% over the standard bath notcontaining the additive over the same 4-hour plating time. The platingrate deviation of the bath containing the 1,3-diisopropylthiourea wasless than .15 mil/hr. over a four-hour plating period, which amounts toabout a loss in plating rate as opposed to approximately 39% loss ofplating rate over the same period displayed by the standard bath withoutthe 1,3- diisopropylthiourea additive. Similarly, the plating ratedeviation of the bath containing the thiobenzanilide was less than .18mil/hr. over a four-hour plating period, which amounts to about an 18.6%loss in plating rate as opposed to approximately 39% loss of platingrate over the same period displayed by the standard bath without thethiobenzanilide additive.

It should be also noted that both the additives of our invention haveincreased the initial plating rate of the optimized preferred bath fom0.77 mil/hr. to 0.965 mil/hr., which amounts to about a increase in theinitial plating rate. correspondingly, the fourth hour plating rateincrease is 75.2% for 1,3-diisopropylthiourea and 67.7% forthiobenzanilide.

The following is a specific example of our invention. A standardsolution comprising 11.8 g./l. of nickel sulfate hexahydrate, 22.3 g./l.sodium hypophosphite monohydrate, 9.34 g./l. acetic acid, 0.88 ml./l.(0.009 g./1.) of an 85% molybdic acid solution, and sodium hydroxide topH of 5.5 was prepared. To one 3.8 liter portion of this solution wasadded 0.012 g./l. of thiobenzanilide. T o a second 3.8 liter portion ofthis solution was added 0.004 g./l. of 1,3-diisopropylthiourea. A third3.8 liter portion of this solution was used as a standard solution withwhich to compare the other solutions. Steel panels (2.54 x 4.39 x 0.66cm.) were plated in these baths. The panels were first prepared forplating by solvent cleaning, cathodic alkaline cleaning (6 volts, 77 C.sec.), rinsing in water, dipping in 3% H 80 for 15 sec., rinsing withwater, and heating the panels to the temperature of the plating bathimmediately prior to immersion therein. This pretreatment was adequatefor the purpose intended and is in no way critical to the effectivenessof the plating step. However, the preheating of the panel prior to itsimmersion into the plating bath was found to be a desirable step in thatbetter adhesion and an initially higher plating rate was obtainable. Apanel was removed and replaced by a fresh panel at the end of each hour.Each panel was weighed before and after plating to determine hourlyplating rates. Agitation was maintained by a reciprocating bar movingthe panels at a rate of 167.6 cm. per min. Though temperatures in theorder of 66 C. to boiling could be used, the baths were maintained at atemperature of 82 C. plus or minus 1 C. The results of these tests areshown in FIGURE 1 wherein curve 1 corresponds to the standard bath andcurves 2 and 3 correspond to the baths having thiobenzanilide and l,3-diisopropylthiourea additives respectively therein. The depositsproduced in all instances were smooth. It was noted that whereas theadditive-free solution produced deposits which were sulfur-free and hada phosphorous content of about 3-5 the deposits produced from the bathshaving the additives of our invention therein contained about 0.005%sulfur and about 6.7-9.6% phosphorous. It was also noted that formaximum effectiveness with the test bath, the area to be plated shouldnot exceed more than about 24 cm. per liter of solution. As the area isincreased, the plating rate decreases. Similarly, as the area decreases,the plating rate increases. However, we have determined that While notcritical, the 24 cm. per liter area to volume ratio is about the mostpractical compromise between plating area and plating rate forproduction purposes.

FIGURE 2 graphically depicts the often erratic results produced by otherapparently chemically similar compounds, some of which have been used bythe prior art to effect other results. These curves were produced in thesame manner as those produced in FIGURE 1. Curve 4 corresponds to theplating rate deviation produced by a 0.00304 g./gal. addition ofZ-mercaptobenzothiazole. Curve 5 corresponds to the plating ratedeviation produced by a 0.00152 g./gal. addition of thioacetamide. Curve6 corresponds to the plating rate deviation produced by a 0.002 g./l.addition of diphenylthioure-a. Curve 7 corresponds to the plating ratedeviation produced by a 0.00076 g./gal. addition of thiourea. Curve 8corresponds to the plating rate deviation produced by a 0.002 g./l.addition of benzylthiourea.

Though we have disclosed our invention solely in terms of one specificembodiment thereof, we do not intend to be limited thereto, except asdefined by the claims which follow.

We claim:

1. In an aqueous electroless nickel plating bath containing nickel ionand hypophosphite ion, the improvement which consists of including insaid bath a small, but effective amount of at least one compoundselected from the group consisting of 1,3-diisopropylthiourea andthiobenzanilide, to increase and stabilize the plating rate of saidbath.

2. The bath as claimed in claim 1 wherein the concentration of said1,3-diisopropylthiourea is 00008-0009 g./l. and the concentration ofsaid thiobenzanilide is 00001-0047 g./l.

3. The bath as claimed in claim 1 wherein the concentration of said1,3-diisopropylthiourea is 0003-0004 g./l., and the concentration ofsaid thiobenzanilide is 0007-0012 g./l.

4. The bath as claimed in claim 3 wherein said bath comprises a nickelion concentration of about 1-15 g./l., a hypophosphite ion concentrationof about 2-40 g./l., at least one buffer selected from the groupconsisting of acetic and propionic acid and having a concentration ofabout 2-30 g./1., and a molybdenum concentration of less than about 0.02g./1.

5. A bath as claimed in claim 4 wherein said bath comprises about11.8-22.3 g./l. of nickel sulfate hexahydrate, 11.8-22.3 g./l. of sodiumhypophosphite monohydlrate, 6.71-l7.2 g./1. of acetic acid, and 0.0088-0.01l6 g./l. of molybdenum, said bath having an initial pH of from4.5-6.0 and a temperature from about 66 C. to boiling.

6. The bath as claimed in claim 5 wherein said bath comprises about 11.8g./l. of nickel sulfate hexahydrate, 22.3 g./l. of sodium hypophosphitemonohydrate, 9.34 ml./l. of acetic acid, 0.009 g./l. of molybdenum, andwherein the concentration of said 1,3-diisopropylthiourea is 0.004g./l., and the concentration of said thiobenzanilide is 0.012 g./l.,said bath having an initial pH of about 5.5 and a temperature of about82 C.

7. In an electroless nickel plating process, the step of immersing thearticles to be plated in a bath as defined in claim 4. Q

8. In an electroless nickel plating process, including the step ofimmersing the articles to be plated in a bath as defined in claim 5.

9. In an electroless nickel plating process, including the step ofimmersing the articles to be plated in a bath as defined in claim 6.

( erences on following page) References Cited UNITED STATES PATENTS 6JULIUS FROME, Primary Examiner L. HAYES, Assistant Examiner Talmey eta1. 117130 Iendrzynski 106-1 US, Cl. X,R Colonel 106-1 XR 5 117-130, 160

Hepfer 106-1 XR

